1. Field of the Invention
The present invention relates to a beam scanning optical system, and more particularly to a beam scanning optical system which is employed in a laser beam printer, a facsimile or the like in order to collect a pencil of rays on a surface of an image medium.
2. Description of Related Art
Generally, a beam scanning optical system suitable for installation in a laser beam printer or a facsimile consists mainly of a semiconductor laser, a deflector such as a polygon mirror and, an f.theta. lens. The deflector is to deflect a pencil of rays emitted from the semiconductor laser at a constant angular velocity, which results in beam scanning (main-scanning). With no optical elements after the deflector, scanning speed varies within a main-scanning line on a beam receiving surface, and quality pictures cannot be obtained. The f.theta. lens is provided in order to prevent such variation in scanning speed. The f.theta. lens is made by combining various concave lenses and convex lenses, and planning such a lens is very complicated. From a viewpoint of manufacture, since there are many surfaces to be ground, it is very hard to improve accuracy, and accordingly the cost is expensive. Moreover, there is a limitation in selecting a material, that is, a light-transmitting material must be selected.
Because of the disadvantages above, it has been proposed to replace the f.theta. lens with an elliptical mirror or a parabolic mirror. However, such mirrors are difficult to manufacture with high manufacturing accuracy.
In view of the problems above, the applicants disclosed in U.S. Pat. No. 4,984,858 an optical system containing a spherical mirror which can be easily manufactured with high accuracy, not an f.theta. lens conventionally proposed mirrors. This arrangement makes the optical system compact, and inhibits curvature of field in a direction perpendicular to the main-scanning direction, and corrects errors of reflecting facets of the deflector in the vertical direction. In this optical system, however, curvature of field and distortion in the main-scanning direction are not satisfactorily inhibited. For example, if priority is given to inhibition of distortion, curvature of field in the main-scanning direction becomes large, and as a result aberration with respect to the main-scanning direction cannot be sufficiently inhibited.